This invention relates generally to rejects separators for fibrous slurries produced by cooking cellulose material such as wood and more particularly to liquid level control method and apparatus for sealed knot separators and knot drainers in a paper pulpmaking operation.
In a pulp mill, a small fraction of the pulp stream consists of undigested wood chips called knots. These knots must be removed from the pulp during processing to negate contamination of the finished pulp with undesirable woody type material. For this purpose, a knot separator is used which may be a one stage system for a low capacity mill or, more commonly, since high capacity knotters reject a considerable amount of good fiber with the knots, a two stage system wherein the rejected knots and fiber from the first stage high capacity knotters are separated again in the secondary or reject stage knot drainer. The two stage system permits a higher capacity operation while, at the same time, improving the efficiency of knot separation from good fiber and knot removal. This improved accuracy results in a decrease in the amount of pulp liquor and fiber being discharged with the knots.
The primary high capacity knotter operates under pressure. It removes the knots from the main pulp stream and concentrates them in a relatively low volume, low consistency rejects flow of concentrated knots with fiber and liquor. The second stage rejects separator drains the pulp fiber and liquor away from the knots, washes adhered fiber off the knots, and rejects fiber free damp knots to a knot tank. This simplifies knot reprocessing or disposal and conserves pulp fiber, liquor and contained chemicals and minimizes undesirable environmental discharges.
A secondary rejects separator or knot drainer is open to the atmosphere through the rejects discharge outlet. Feed rate to the knot drainer is determined by the knot content of the cooked pulp and the operation of the pulp system and is subject to wide fluctuations. It is, therefore, possible for the knot drainer to either overflow or "run dry", depending upon the feed flow rate of knots, fiber, and liquor in combination. The knot drainer does not actually run dry, but this terminology is used to describe a condition which occurs due to the differential pressure (i.e., delta P) across the screen basket reaching a level at which the inlet side of the apertures in the screen basket are incipiently blinded over and thus fiber and/or liquor incipiently ceases to flow through the apertures at which point the liquid level in the accepts chamber of the knot drainer may, if not maintained full, fall below the top of the separating screen basket and allow the ingress of air into the accepts compartment thereby maximizing delta P. This will cause blinding off of the separating screen basket apertures and may require shutdown of the knot drainer for cleanout and the shutdown of the entire pulping line during the cleanout period. The loss of production due to running dry and the loss of fiber and pulp liquor due to overflow are significant economic risks.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.